To express colors, there are methods using a color system of psycho-physical color (for example, XYZ color system) and methods using a color system of perceived color (for example, L*a*b* color system). Further, regarding color of light reflected by an object (object color), a method of correlating these color systems with each other by calculation has been known (see, for example, non-patent document 1).
For example, the following calculation equations (21) to (23) are used to correlate the XYZ color system and the L*a*b* color system with each other. X0, Y0, Z0 are tristimulus values (Y0=100) of color of a perfect reflecting diffuser illuminated by standard light. The standard light is light whose spectral distribution is known (for example, light D65). X, Y, and Z represent tristimulus values of color of an object illuminated by the same standard light, L* represents a Munsell value (whiteness), a* represents perceived chromaticity in which hue and chroma of red-green are taken into consideration, and b* represents perceived chromaticity in which hue and chroma of yellow-blue are taken into consideration.L*=116(Y/Y0)1/3  (21)a*=500[(X/X0)1/3−(Y/Y0)1/3]  (22)b*=200[(Y/Y0)1/3−(Z/Z0)1/3]  (23)
Non-patent document 1: CIE Pub. No. 15.2 (1986): Colorimetry. 3rd ed., Bureau Central de la CIE.